Bimetal tibial component construct for knee joint prosthesis

REFERENCE TO PENDING PRIOR APPLICATIONS

0001This application:

0002(1) is a continuation of pending prior U.S. patent application Ser. No. 09/910,646, now U.S. Pat. No. 6,652,588, filed Jul. 19, 2001 by Daniel E. E. Hayes, Jr. et al. for BIMETAL TIBIAL COMPONENT CONSTRUCT FOR KNEE JOINT PROSTHESIS; and

0003(2) is a continuation of pending prior U.S. patent application Ser. No. 09/901,310, filed Jul. 09, 2001 by Alfred S. Despres III et al. for IMPLANT WITH COMPOSITE COATING which is in turn a continuation of prior U.S. patent application Ser. No. 09/079,502, now U.S. Pat. No. 6,261,322, filed May 14, 1998 by Alfred S. Despres III et al. for IMPLANT WITH COMPOSITE COATING, and claims the benefit of prior U.S. Provisional Patent Application Ser. No. 60/219,961, filed Jul. 20, 2000 by Daniel E. E. Hayes, Jr. et al. for BIMETAL TIBIAL COMPONENT CONSTRUCT.

0004The four above-identified patent applications are hereby incorporated herein by reference.

FIELD OF THE INVENTION

0005This invention relates to surgical apparatus and procedures in general, and more particularly to orthopedic prostheses for restoring the knee joint.

BACKGROUND OF THE INVENTION

0006Joint replacement surgery seeks to replace portions of a joint with prosthetic components so as to provide long-lasting function and pain-free mobility.

0007For example, in the case of a prosthetic total hip joint, the head of the femur is replaced with a prosthetic femoral stem component, and the socket of the acetabulum is replaced by a prosthetic acetabular cup component, whereby to provide a prosthetic total hip joint.

0008In the case of a prosthetic total knee joint, the top of the tibia is replaced by a prosthetic tibial component, and the bottom of the femur is replaced by a prosthetic femoral component, whereby to provide a prosthetic total knee joint.

0009The present invention is directed to orthopedic prostheses for restoring the knee joint and, in particular, to improved prosthetic tibial components.

0010There is a long and varied history in the use of different materials for joint replacement prostheses. Some early attempts, such as stainless steel hip prostheses, were found to be reasonably successful and are still in use today. Other attempts, such as acrylic femoral head replacements or Teflon “TMJ” replacements, were found to be unacceptable and have been abandoned.

0011Currently, combinations of materials are generally used to form joint replacement prostheses.

0012More particularly, in the case of a prosthetic total hip joint, the prosthetic femoral stem component typically comprises a metal, and the prosthetic acetabular cup component typically comprises a metal seat with a plastic liner.

0013In the case of a prosthetic total knee joint, the prosthetic tibial component typically comprises a metal base topped with a plastic bearing surface, and the prosthetic femoral component typically comprises a metal.

0014The present state of the art is currently dominated by the use of three different materials: titanium and its alloys, cobalt-based alloys and polyethylene plastics. The two metallic materials are generally used for structural constructs (e.g., constructs that must carry the loads transmitted through the joint), and polyethylene is generally used as a bearing material in the joints (e.g., to slide or rotate against an opposing metallic component).

0015Ceramic bearing couples have also been used in the art to some extent, but their use is relatively limited due to price and strength considerations.

0016The vast majority of structural implant constructs are currently made from either titanium alloys (e.g., Ti6Al4V) or cobalt-based alloys (e.g. CoCr alloys, including CoCrMo alloys). These materials have different advantages and disadvantages.

0017More particularly, titanium alloys generally exhibit relatively high general fatigue strength, relatively low stiffness compared to alternative materials, and excellent biocompatibility properties. Titanium alloys, however, also tend to suffer from notch sensitivity in fatigue, which significantly reduces the fatigue strength of the construct when the surface is notched, roughened or porous-coated. Titanium alloys are also prone to scratching and make relatively poor sliding couples with polyethylene.

0018CoCr alloys generally have relatively high fatigue strengths, are relatively notch insensitive, and are relatively tough and resistant to scratching, thus making them excellent candidates for forming sliding couples with polyethylene. However, CoCr alloys are also relatively stiff, which can cause load pattern problems when coupled with flexible human bones, and they are not as biocompatible as many other alloys due to their chrome, and in some cases nickel, content.

0019In the 1980's, titanium alloys were used in many applications to take advantage of their biocompatibility. However, the applications that included sliding surfaces, such as femoral heads for the hip and knee femoral components, tended to have significant-problems with wear debris and scratching, and many exhibited clinical failure.

0020From this experience, implants were developed that combined the two aforementioned materials (i.e., titanium and CoCr alloys) in advantageous ways.

0021One early product was a knee femoral component that had a sliding surface of CoCr and a bone ingrowth surface of titanium. This design took advantage of CoCr's excellent wear characteristics in sliding articulations with the tibial component's polyethylene bearing, while still providing excellent bone ingrowth at the bone/prosthesis junction.

0022The aforementioned two materials (i.e., titanium and CoCr alloys) have also been used on hip femoral stem components. More particularly, hip femoral stem components have been developed which comprise an inner core of CoCr covered with a coating of titanium for bone ingrowth. This layered construction is desirable because stems made entirely of titanium, with titanium ingrowth surfaces, are too weak, while stems that are made entirely of CoCr, with CoCr ingrowth surfaces, do not have adequate biocompatibility. The combination of these two materials in a single construct provides a stem that is strong enough and also has a good bone ingrowth surface.

0023Another attempt to improve the biocompatibility of the bone ingrowth surface has been to coat the surface with hydroxyapatite (HA). However, HA, while it yields excellent short term results, has problems with long term stability due to its pH sensitivity. More particularly, the pH of the body may fluctuate due to a variety of conditions such as nutrition and disease, and this can undermine the effectiveness of HA bone ingrowth surface.

0024Another attempt to increase the hardness of the articulating surface has been to coat the articulating surface with a ceramic such as titanium nitride. The main limitation to this approach is that loading and abrading tend to undermine the mechanical integrity of the union between the ceramic coating and the substrate, and this can lead to prosthesis failure.

0025As wear issues relating to the main articulating surfaces have been addressed and incidences of gross and catastrophic wear eliminated, it has been discovered that the locking interface between the polyethylene bearing construct and the metal base construct can also be a significant source of wear debris. More particularly, it has been discovered that sliding motions in the junction between the polyethylene bearing construct and the metal base construct produce particles of polyethylene that can migrate out of the joint and into the body. Small abrasive particles can also migrate into the interface between the polyethylene bearing construct and the metal base construct and scratch the metal base construct, particularly where the metal base construct is formed out of titanium. This issue of “backside wear” has been a significant issue for research and debate over the last five years or so.

0026Attempts to address this issue have, to date, been limited to polishing the titanium mating surface of the metal base construct, as disclosed in U.S. Pat. No. 5,310,408 and as practiced in the “Reflection Cup” product marketed by Smith + Nephew of Memphis, Tenn. However, polishing a titanium surface has not worked well in previous attempts in sliding couples (i.e., in the femoral head component of a prosthetic total hip and in the prosthetic femoral component of a prosthetic total knee), and it has had only limited success in reducing wear debris at the locking interface between the polyethylene bearing construct and the metal base construct. This is primarily due to the inherent material limitations of the titanium metal base construct in the polished locking mechanism configuration.

0027No existing metallic construct that assembles with a polyethylene bearing is made of two metals (i.e., is bimetallic).

0028No existing bimetallic constructs lock with polyethylene.

SUMMARY OF THE INVENTION

0029This invention provides for a novel orthopedic prosthesis, specifically a prosthetic tibial component for a prosthetic total knee joint, that comprises two constructs, one being a metal base construct that engages the bone and the other being a polyethylene bearing construct that attaches to the metal base construct and articulates with a prosthetic femoral component on the opposing side of the joint. The metal base construct is composed of two different metals, one of which engages the bone surface and the other of which engages the polyethylene bearing construct. Each of these metals is selected so that its characteristics are well suited to its particular function. More particularly, the first metal (i.e., the one that engages the bone surface) is selected so as to provide a superior bone-engaging face, while the second metal (i.e., the one that engages the polyethylene bearing construct) is selected so as to provide a superior polyethylene-engaging face. By combining the different material characteristics of two different metals in the metal base construct, it is possible to simultaneously form a superior bone-engaging face and a superior polyethylene-engaging face. Among other things, by selecting two appropriate metals for the metal base construct, superior bone ingrowth can be achieved while still avoiding the aforementioned problems with “backside wear”.

BRIEF DESCRIPTION OF THE DRAWINGS

0030These and other objects and features of the present invention will be more fully disclosed or rendered obvious by the following detailed description of the preferred embodiments of the invention, which is to be considered together with the accompanying drawings wherein like numbers refer to like parts and further wherein:

0031<figref idref="DRAWINGS">FIG. 1</figref> is a schematic side view showing a prosthetic total knee joint positioned within a patient's body;

0032<figref idref="DRAWINGS">FIG. 2</figref> is an exploded schematic view showing the prosthetic tibial component of the prosthetic total knee joint shown in <figref idref="DRAWINGS">FIG. 1</figref>; and

0033<figref idref="DRAWINGS">FIG. 3</figref> is a schematic side view of the prosthetic tibial component of the prosthetic total knee joint shown in <figref idref="DRAWINGS">FIG. 1</figref>.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

0034Looking first at <figref idref="DRAWINGS">FIG. 1</figref>, there is shown a prosthetic total knee joint <b>5</b> which generally comprises a prosthetic tibial component <b>10</b> secured to the top end of a resected tibia <b>15</b>, and a prosthetic femoral component <b>20</b> which is secured to the bottom end of a resected femur <b>25</b>.

0035Prosthetic tibial component <b>10</b> is shown in greater detail in <figref idref="DRAWINGS">FIGS. 2 and 3</figref>. Prosthetic tibial component <b>10</b> generally comprises a metal base construct <b>30</b> and a polyethylene bearing construct <b>35</b>.

0036More particularly, metal base construct <b>30</b> comprises a base plate <b>40</b>, a stem <b>45</b> and a plurality of pegs <b>50</b> descending from base plate <b>40</b>, a plurality of screws <b>55</b> passing through base plate <b>40</b>, a pair of rails <b>60</b> running along the top surface of base plate <b>40</b> and defining a groove <b>65</b> therebetween, and a pair of end walls <b>70</b> connected to base plate <b>40</b>.

0037Polyethylene bearing construct <b>35</b> comprises a flat bottom surface <b>75</b> having a longitudinally-extending recess <b>80</b> in which is disposed a longitudinally-extending tongue <b>85</b>. Tongue <b>85</b> is sized to slidingly fit in the groove <b>65</b> of metal base construct <b>30</b>, whereby polyethylene bearing construct <b>35</b> may be secured to metal base construct <b>30</b>.

0038In use, the top end of tibia <b>15</b> is resected, metal base construct <b>30</b> is secured to tibia <b>15</b> via screws <b>55</b>, and polyethylene bearing construct <b>35</b> has its tongue <b>85</b> slid into groove <b>65</b> until polyethylene bearing construct <b>35</b> engages the base plate's end walls <b>70</b>.

0039In accordance with the present invention, metal base construct <b>30</b> is formed with a bimetal construction. More particularly, the metal base construct <b>30</b> is composed of two different metals, a first metal <b>87</b> which engages tibia <b>15</b> and a second metal <b>90</b> which engages polyethylene bearing construct <b>35</b>. Each of these metals is selected so that its characteristics are well suited to its particular function. More particularly, first metal <b>87</b> (i.e., the one that engages tibia <b>15</b>) is selected so as to provide a superior bone-engaging face, while second metal <b>90</b> (i.e., the one that engages polyethylene bearing construct <b>35</b>) is selected so as to provide a superior polyethylene-engaging face. By combining the different material characteristics of two different metals in base metal construct <b>30</b>, it is possible to simultaneously form a superior bone-engaging face and a superior polyethylene-engaging face. Among other things, by selecting two appropriate metals for the metal base construct, superior bone ingrowth can be achieved while still avoiding the aforementioned problems with “backside wear”.

0040For instance, a base metal construct <b>30</b> may be formed whose bone-engaging surfaces are formed from titanium and whose polyethylene-engaging surfaces are formed from CoCrMo. This construction places a good bone ingrowth metal against the bone and a good polyethylene-engaging metal against the polyethylene, whereby to provide a significantly superior prosthetic tibial component <b>10</b>.

0041It is also possible to use other metals that are suitable in both strength, biocompatibility, and joinability to make the bimetal tibial component construct.

0042By way of example but not limitation, first metal <b>87</b> may comprise titanium, titanium alloys, tantalum, tantalum alloys or other metals and/or metal alloys consistent with the present invention. Among other things, first metal <b>87</b> is preferably a material which is highly biocompatible and which exhibits good bone ingrowth properties.

0043By way of further example but not limitation, second metal <b>90</b> may comprise CoCrMo, cobalt based alloys, stainless steels, zirconium based alloys or other metals and/or metal alloys consistent with the present invention. Among other things, second metal <b>90</b> is preferably a material which has relatively high hardness and which is scratch resistant.

0044For the purposes of the present invention, the term bimetal may be defined as a composite of two metals, where each of the metals has a different primary constituent. The bimetal construct can be formed from two different commercially pure metals, from two alloys of different base metals, or a combination thereof.

0045The bimetal construct can be fabricated using a variety of techniques. In one preferred form of the invention, the bimetal construct is fabricated using the method disclosed in pending U.S. patent application Ser. No. 09/079,502, which patent application is hereby incorporated herein by reference. Alternatively, the bimetal construct can be fabricated by other techniques such as plasma spray, diffusion bonding, sintering, or metallurgical methods, e.g., such as a method of the sort disclosed in U.S. Pat. No. 5,323,954 (Shetty).